1. Field of the Invention
The present invention relates to a semiconductor device, that is, an RFID tag, which transmits and receives data by wireless communication. In particular, the present invention relates to a protection circuit which prevents a wireless communication circuit from being damaged or prevents elements provided in the circuit from deteriorating and being damaged when the RFID tag receives a signal having excessive electric power in wireless communication.
2. Description of the Related Art
In recent years, an individual identification technology using wireless communication (hereinafter referred to as a wireless communication system) has attracted attention. In particular, a wireless communication system in which a tag utilizing an RFID (radio frequency identification) technology (hereinafter referred to as an RFID tag regardless of its shape such as a card shape or a chip shape) is used as a data carrier which transmits and receives data by wireless communication has attracted attention. An RFID tag is also referred to as an IC tag, a wireless tag, or an electronic tag.
In the field of manufacture and distribution, a wireless communication system with an RFID tag is utilized for management of a large number of items and the like instead of conventional management with a bar code, and is applied for individual identification.
Here, a wireless communication system refers to a communication system in which data is transmitted and received wirelessly between a transmitter receiver (also referred to as an interrogator), such as a reader/writer (hereinafter referred to as a R/W), and an RFID tag. In such a wireless communication system, data which is to be transmitted and received is superimposed on carrier waves emitted from the R/W.
The RFID tag is categorized into an active-type and a passive-type. A passive RFID tag does not include a battery. Drive electric power for a circuit in an RFID tag is supplied by generating DC voltage for drive in the RFID tag from a carrier wave emitted from a R/W or from an amplitude-modulated wave generated by superimposing a modulated wave on a carrier wave. On the other hand, an active RFID tag includes a battery for supplying electric power which allows the battery to operate.
A passive RFID tag can be made smaller and lighter because it does not include a battery; however, the passive-type RFID tag cannot generate electric power for circuit operation when it receives less carrier waves than required because it generates drive electric power by receiving a carrier wave emitted from a R/W.
Such a wireless communication system needs to satisfy the following conditions: the long maximum communication distance and recognition of all the RFID tags when a number of RFID tags are provided in a particular area.
Both of the conditions can be satisfied by increasing electric power in a carrier wave from a R/W. Therefore, in the case where an RFID tag is provided in the close vicinity of the R/W or in the case where a small number of RFID tags are provided in the vicinity of the R/W, the RFID tag receives a signal having quite high electric power.
When an RFID tag receives a signal having excessive electric power, a DC voltage generated in the RFID tag also becomes excess. When high voltage is applied to an internal circuit in the RFID tag, elements included in the internal circuit are damaged. Therefore, in an RFID tag used for a wireless communication system in which a carrier wave from a R/W has high electric power, a protection circuit is provided so that excessive electric power is not generated in the tag in the case where the RFID tag is in circumstances where the RFID tag receives a signal having excessive electric power (see Patent Document 1 and Patent Document 2).